Shore Fly

Placopsidella marquesana (Malloch)

Mosillus marquesana Malloch, 1933:13.

Gymnopa marquesana.—Cresson, 1945:54 [combination: species unrecognized].

Placopsidella cynocephala sensu authors [misidentification].—Adachi, 1952:354 [misidentification provided by Wirth (in litt.); listed under family Canacidae].—Hardy, 1952:446 [repeat of information cited by Adachi, 1952].—Tenorio, 1980:270–271, 350–351 [figures of female ventral receptacle, larva, and puparium].

DIAGNOSTIC DESCRIPTION.—Moderately small to medium-sized shore flies, length 2.65–3.30 mm.

Head: Median facial carina usually short and wide, 2 times higher than wide, surface appearing shiny, at most microsculptured, lacking conspicuous transverse rugae, lateral margins more regular, shallowly convex; yellowish gray microtomentose portion of fronto-orbit extended dorsad from parafacials to larger lateroreclinate fronto-orbital seta; microtomentum of fronto-orbits with indistinct and generally brownish coloration immediately around base of fronto-orbital setae only, not traversing width of fronto-orbit, otherwise whitish to yellowish gray. Eye-to-cheek ratio 0.37.

Thorax: Scutellum with posteroapical margin narrowly rounded to triangular, wider than long; scutellar ratio 0.90. Knob of halter yellowish to white. Costal vein ratio 0.50; M vein ratio 0.63.

Abdomen: Male terminalia (Figures 5, 6) as follows: epandrium plus fused surstyli in lateral view with width more or less uniform throughout length, less than greatest width of cerci in profile, ventral portion slightly and gradually narrower; cerci in posterior view shaped like ½ of an oval with median margin nearly straight except for subventral, shallowly rounded flange, dorsal ½ wider than ventral ½ ventral angle more acutely angulate, cerci in lateral view more or less elliptical, with anterior margin nearly straight; gonite in lateral view long and narrow and with a ventrobasal triangular sclerite between extended arm and hypandrium, sides of extended gonal arm more uniformly parallel and lacking prominent preapical flange on anterior surface, apex of arm reflexed anteriorly and acutely pointed; aedeagus in lateral view with basal ½ gradually becoming narrower, apical ½ nearly parallel sided and apex slightly curved anteriorly and bluntly rounded, aedeagus in dorsal view moderately narrowly subtriangular, basal width slightly more than double length, apex bluntly rounded.

TYPE MATERIAL.—The holotype male is labeled “Tahauku, HivaOa VII–10–29 [10 Jul 1929, date handwritten]/sea shore/Marquesas Islands/Mumford Adamson/Pacific Entomo-logical Survey/Placopsidella cynocephala Kertész 150. [1950?] det WWirth [handwritten except for “det WWirth”; black submargin]/Mosillus marquesana Type det. JRMALLOCH [species epithet and “Type” handwritten; black submargin].” The holotype is double mounted (glued to a paper point), is in good condition, and is in the B.P. Bishop Museum, Honolulu, Hawaii.

OTHER SPECIMENS EXAMINED.—AMERICAN SAMOA. Tutuila: Auasi, 10 Dec 1953, C. Hoyt (5, 2; BBM, USNM); Pago Pago, 7–11 Oct 1963, N.R. Spencer (7, 17; BBM); Fagatogo, 2 Sep 29 Jun 1963–1964, N.R. Spencer (10, 27; BBM); Malinua, 7 Jan 1964, N.R. Spencer (1; BBM); Tapuna, 13 Oct 1964, N.R. Spencer (1; BBM); Taputimu, 19 Aug–7 Nov 1963–1964, N.R. Spencer (3, 5; BBM). LINE ISLANDS. Palmyra Island, Feb 1948, N.L.H. Krauss (2, 1; USNM); Line Island, Jul 1962, M.R. Wheeler (1, 1; USNM). FRENCH POLYNESIA. Society Islands: Bora Bora Island, Apr 1961, J.N. Belkin (9; USNM); Tahiti Island, Papeete, Blue Lagoon, 4 Mar 1934, E.C. Zimmerman (3, 4; BBM, USNM); Arihiri, Pare, 14 Mar 1934, E.C. Zimmerman (1?; BBM). HAWAIIAN ISLANDS. Hawaii: Hilo, 6 Jul 1968, G. Steyskal (2, 2; USNM); Hilo Bay, mouth of Wailuku River, 30 May 1970, J.A. Tenorio (6, 2; UHH); Kealakekua, 12 May 1959, S. Kimoto (2; BBM). Kauai: Nawiliwili Dock, 14 Jul 1968, J.A. Tenorio (1; UHH). Maui: Hana, Aug, N.L.H. Krauss (3, 1; BBM). Oahu: Aiea, Mar 1959, D.E. Hardy (23, 5; UHH, HNHM, USNM); Haleiwa, Sep 1955 (2; UHH); Hanauma Bay, 6–7 Mar 1952–1961, M.S. Adachi, A.L. Melander, L.W. Quate (14, 7; BBM, UHH, USNM); Honolulu, 10 Nov–19 Dec 1941–1952 (some specimens intercepted from planes), R.W. Cox, C.R. Joyce (3, 8; UHH, USNM); Waikiki (on dead fish), Mar 1957, M.S. Adachi (1, 1; UHH). Wake Island: 21 Nov 1959 (on dead fish) (2 4; USNM). SOLOMON ISLANDS. Florida Island, Mar 1945, G.E. Bohart (1, 8; USNM).

DISTRIBUTION (Figure 7).—Marquesas Islands, American Samoa, French Polynesia, Hawaiian Islands, Solomon Islands, and Line Islands.

NATURAL HISTORY.—Tenorio (1980) provided information about this species from observations in the field and laboratory. This species occurs commonly in seaweed and other debris, especially sugar cane debris, on beaches of the Hawaiian Islands. Although specific experiments on food ingestion were not conducted, Tenorio successfully bred this species on a medium of seaweed that was maintained in gallon jars. Whether the flies fed directly on the seaweed or as scavengers on crustacea or other animals among the seaweed is not known. The trophic requirements of the genus, as a consequence, remain unresolved, but the leads of Tenorio coupled with the data under P. cycnocephala (page 10) ought to provide good possibilities for future studies.

Based on laboratory rearings of P. marquesana, Tenorio (1980:351) reported the following schedule for the life cycle of this species: eggs hatched in about 24 hours; larval development ranged from 8 to 10 days; and the puparial period was 10–13 days. A complete life cycle, egg to adult, would span 19–23 days.

Tenorio's description (here included with Figures 8–12) of the third instar larva and puparium is as follows (1980:350–351; original reference to figures has been deleted):

Mature larvae and puparia about 4 mm in length.

Larvae: White; body segments with lateral margins dorsally and ventrally expanded into pointed spines, these spines difficult to see in the white larva, but very conspicuous on puparium; smaller tubercles are present on dorsal and ventral surfaces of segments. Anterior spiracles not externally evident in larva. Posterior spiracular tubes dorsoapical, very short, with three spiracular openings; four groups of palmate hairs border the openings, the hairs reaching to about the rim of the spiracular tube. Cephalopharyngeal skeleton as…figure[d].

Puparia: Yellow, spines as described for the larva, every evident on the puparium. Anterior spiracles conspicuous, borne on flattened disc, rimmed with about 14 to 15 digits.